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What is Spectroscopy?
■ Spectroscopy is a necessary tool for structure determination.
■ Organic chemists use spectroscopy as a necessary tool.
■ The first spectroscope was invented in 1859 by the German chemist
Robert Wilhelm Bunsen and the German physicist Gustav Robert
■ Spectroscopy may be defined as the study of the quantized
interaction of electromagnetic radiations with matter.
Difference between spectrometer and
■ SPECTROMETER: An optical spectrometer ( spectrograph or
spectroscope) is an instrument used to measure properties of light over
a specific portion of the electromagnetic spectrum, typically used in
spectroscopic analysis to identify materials.
■ SEPCTROPHOTOMETER: A spectrophotometer is an analytical
instrument used to quantitatively measure the transmission or
reflection of visible light, UV light or infrared light.
What is electromagnetic spectrum?
■ Electromagnetic spectrum covers a very wide range of electromagnetic
radiations from cosmic rays to radio waves at the other end.
■ The arrangement of all types of electromagnetic radiations in order of
their wavelengths or frequencies is known as complete electromagnetic
Ultraviolet (UV) andVisible
■ Ultraviolet (UV) andVisible Spectroscopy deals with the recording of
the absorption of radiations in the ultraviolet and visible regions of the
■ The ultraviolet region extends from 10 to 400 nm.
■ Near ultraviolet (quartz) region (200- 400 nm)
■ Far or vacuum ultraviolet region (10- 200nm)
■ The visible region extends from 400 to 800 nm.
Principle of UV-Vis spectroscopy
■ UV-Visible spectroscopy follows the Beer- Lambert Law.
■ Beer’s Law
The intensity of a beam of monochromatic light decreases exponentially with the
increase in concentration of the absorbing substance.
■ Lambert’s Law
When a beam of light is allowed to pass through a medium, the rate of decrease
of intensity with the thickness of medium is directly proportional to the intensity of the
■ Radiation source: Hydrogen- discharge lamp is the most commonly
used source of radiation in the UV region. A deuterium- discharge lamp
is used in its place when more intensity is desired.
■ Monochromator: It disperses the radiations obtained from the source
into their separate wavelengths.
– Grating made up of quartz
■ Detectors: These have photocells or photo multiplier tubes which
generate voltage proportional to the radiation energy that strikes them.
■ Amplifier : The spectrometer has balancing electronic amplifier which
subtracts the absorption of the solvent from that of the solution
■ Recorder : It automatically records the spectrum as a plot of
wavelengths of absorbed radiations against absorbance or molar
■ UV- visible spectra are usually recorded either in very dilute solutions or
in the vapour phase.
■ The sample is dissolved in some suitable solvent which does not itself
absorb radiation in the region under investigation.
■ Commonly used solvents are cyclohexane, 1, 4- dioxane, water and 95%
■ The chosen solvent should be inert to the sample.
Theory of U-VVisible Spectroscopy
■ U-V visible absorption spectra originate from electronic transitions
within a molecule.
■ These transitions involving promotion of valence electrons from the
ground state to the higher-energy state(excited state) are called
electronic excitations and are caused by the absorption of radiation
energy in the UV-visible regions of the electromagnetic spectrum.
■ Since various energy levels of molecules are quantized, a particular
electronic excitation occurs only by the absorption of specific
wavelength of radiation corresponding to the required quantum of
■ According to molecular orbital theory, the excitation of a molecule by
the absorption of radiation in the UV-visible regions involves promotion
of its electrons from a bonding, or non bonding (n)orbital to an
■ σ - σ * transition
The transition or promotion of an electron from a bonding sigma
orbital to the associated antibonding sigma orbital is σ - σ * transition. It is
a high energy process because σ bonds are generally very strong.
■ n - σ * transition
Transition or promotion of an electron from a non-bonding orbital
to an antibonding sigma orbital is designated as n - σ * transition.
Compounds containing non bonding electrons on a heteroatom are
capable of absorption due to n - σ *Transitions.These transitions require
lower energy than σ- σ* transitions
■ π- π* transition
The transition or promotion of an electron from a π bonding orbital
to a π antibonding orbital is designated π- π* transition.These type of
transitions occur in compounds containing one or more covalently
unsaturated groups like C=C,C=O,NO2 etc., π- π*Transitions require lower
energy than n - σ * transitions.
■ n - π* transition
The transition or promotion of an electron from a non-bonding
orbital to a π antibonding orbital is designated n - π*.This transition
reqires lowest energy.
Formation of Absorption Bands
■ Since the energy required for each electronic transition is quantized, the
UV-visible spectrum is expected to exhibit a single, discrete line
corresponding to each electronic transition.
■ Broad absorption bands are usually absorbed.
Designation of Absorption Bands
■ UV-visible absorption bands may be designated by the type of
electronic transition from which they originate.
■ K-Bands: These bands originate from π- π* transitions in compounds
having π- π* conjugated system.
■ R-Bands:These bands originate from n - π* transitions of a single
■ B-Bands:These bands originate from π- π* transitions in aromatic or
■ E-Bands: Similar to B-Bands , these are characteristic of aromatic and
heteroaromatic compounds and originate from π- π* transitions of the
ethylenic bonds present in the aromatic ring.
Absorption and Intensity Shifts
■ Bathochromic Shift or Effect. The shift of an absorption maximum to a longer wavelength
due to the presence of an auxochrome or solvent effect is called a bathochromic shift or red
■ Hypsochromic Shift or Effect. The shift of an absorption maximum to a shorter
wavelength is called hypsochromic or blue shift.This is caused by the removal of
conjunction or change in the solvent polarity.
■ Hyperchromic Effect: An effect which leads to an increase in absorption intensity Emax is
called hyperchromic effect.The introduction of an auxochrome usually causes
■ Hypochromic Effect: An effect which leads to a decrease in absorption intensity Emax is
called hypochromic effect.This is caused by the introduction of a group which distorts the
Applications of Ultraviolet andVisible
■ Detection of a functional group (Chromophore).
■ Detection of Conjugation and Elucidation of its nature.
■ Study of Extent of Conjugation.
■ Distinction between Conjugated and Unconjugated Compounds.
■ Study of Strain.
■ Determination of Configurations of Geometrical isomers.
■ Study ofTautomerism.
■ Confirmation of Suspected Phenols and Aromatic Amines.
■ Study of Structural Features in Different Solvents.